Is it possible that if a light beam went out into space, it could travel along a curved path and end up back where it started, because of the curvature of spacetime? Does this mean that we can look into space and see light from Earth from long ago? Can we look back and see ourselves?

This analogy is frequently used to describe what the 'shape' of the Universe might be, however the analogy is a little flawed, and so drawing conclusions based on it will likely lead you down the wrong path.

Firstly, as far as we can measure, our Universe is consistent with being flat. This means that rays of light would just travel off in a straight line forever and never come back. This measurement has slight errors associated with it, so it is still possible that our Universe has either a just barely 'open' or 'closed' geometry. In an open universe, parallel rays of light would diverge as they travel and would never return. In a closed universe, rays of light would eventually return, if the expansion was slow enough.

If our Universe were just closed, light would still never return to its original position. The expansion rate in our Universe is sufficiently rapid that the light will never be able to return to its original position. Instead it will just keep getting further from the source, forever. This has been true for our Universe since the beginning of inflation, a fraction of a second after the Big Bang, but may not have been before then.

You could imagine a universe that is different from ours, where the expansion rate is slower. In such a universe, light rays might be able to return to the source, if such a universe didn't recollapse before they could make it all the way around.

About the Author

Mike is a fourth year astronomy graduate student at Cornell, where he works with Professors Martha Haynes and Riccardo Giovanelli on the ALFALFA survey, a blind survey of gas-rich galaxies in the local Universe carried out with the 305m Arecibo telescope in Puerto Rico.

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